1. Field of the Invention
The invention relates to an electromagnetic flowmeter having a measuring tube, and more particularly, to an electromagnetic flowmeter having a measuring tube with an electrically-insulating internal coating positioned in a recess of the measuring tube.
2. Description of Related Art
The measurement principle of electromagnetic flowmeters in based on the separation of moving charges in a magnetic field. Electromagnetic flowmeters are generally formed of a measuring tube of a non-magnetic substance e.g., of a non-magnetic metal that is electrically insulated by an insulating coating from a measurement fluid on a flow-side and is permeated by a magnetic field perpendicular to a direction of flow created by a magnetic device e.g., coils. If the measurement fluid flowing through the measuring tube has a minimum conductivity, charge carriers, e.g., ions or charged particles, present in the conductive measurement fluid are deflected by the magnetic field. A voltage is created by the charge separation on measurement electrodes arranged perpendicular to the magnetic field and to the direction of flow, which is detected by a measuring device and evaluated as a measurement voltage. The value of the measurement voltage is proportional to the flow speed of the charge carriers moving within the measurement fluid.
The electrically insulating coating consists of different electrically insulating materials in the related art. For example, the electrically insulating coating comprises plastics or ceramics and is applied by means of conventional coating methods.
For example, European Patent Application EP 1285235B1 and corresponding International Patent Application Publication WO 01/90702 A2, disclose an electromagnetic flowmeter in which the electrically insulating coating is applied either on an entire inner surface of the measuring tube or, alternatively, only partially as a circular ring on the measuring tube in the area of the electrodes. However, the electrically insulating coating known from the related art has the disadvantage that the electrically insulating coating can be damaged and possibly dissolved by the flowing medium. In particular, the electrically insulating coating can be damaged and possibly dissolved at transition edges between the measuring tube and the electrically insulating coating.